Tannic Acid

¶ Tannic Acid. A brownish white extract from gall-nuts and oak bark (tanbark). Tannic acid and pyro-gallic acid, which is also an extract from gallnuts, are the basis for brown stains. These acids are diluted with water, brushed on and allowed to dry. Then usually a strong alkaline solution is brushed on. The colors produced with these acids are natural chemical changes in the wood and are absolutely permanent in strong light. The strength of the solutions used, of course, governs the shade of color gained. Most woods contain tannic acid, or tannin, as it is sometimes called. When a wood is deficient in the amount of tannic acid it contains a wash of tannic acid in water enables the finisher to produce a much deeper color or a different one.

¶ Tannic acid is not present in the same amount in all trees, nor in all boards. Consequently a finisher cannot make a uniform finished color on mixed lumber, that is, boards from many trees by using either acid or alkaline solutions. Furniture factories select wood to match up as nearly as possible when chemical stains are used. To offset the deficiency of tannic acid in some surfaces some finishers, who are working on surfaces like furniture where a uniform color is essential, wash over the wood first with a 5% solution of tannic acid, or a 3% solution of pyrogallie acid (5 ounces tannic acid and 95 ounces of water, or 3 ounces of pyrogallic acid and 97 ounces of water). Then for the brawn stains the wood is coated, after the tannic acid wash is dry, with a solution of potash, soda or ammonia and a uniform color results. The light-colored boards are coated more than once with the tannic acid solution, however, to bring them up to the color of the dark boards. One formula for the alkaline wash of potash sometimes used reads: 2 ounces caustic potash, 4 ounces bichromate of potash and 1 gallon of water.

Next Page: Nitric Acid.